This invention relates to drying equipment and processes, and more particularly, to a novel drying apparatus and process in which the material to be dried is atomized and dried by the pulsating flow of a stream of hot gases.
The general process known as combustion drying has been in use for many years. The process has been widely used to remove moisture to obtain or recover a solid material which has been in suspension or solution in a fluid. Typically, the fluid is atomized and the resultant spray is subjected to a flow of hot gases from a combustion process such as that available from an air heater or a pulse combustor to evaporate the moisture from the spray. The solid particles are then carried from the drying chamber by the flow of the drying gas and are removed from the gas by means such as a cyclone separator.
There are two types of pulse combustors. The first is the valved type pulse combustor of which the V-1 "Buzz Bomb" engine is the best known example. The second is the air valved pulse combustor which uses the pulse energy to pump its combustion air. The best known example of this type is the air valve engine developed by Mr. Raymond Lockwood and disclosed in many of his patents such as U.S. Pat. No. 3,462,955.
An air valved pulse combustor consists of a combustion chamber where the fuel is introduced, a combustion chamber inlet which is a short tube, and a combustion chamber tail pipe which is longer than the combustion chamber inlet. Fuel is pressure atomized in the combustion chamber, and when the proper explosive mixture is reached, a spark plug ignites it for initial starting. The fuel and air explode and burning gas expands out both the inlet tube and the outlet tube. The energy released in the explosion provides thrust or power when the two shock waves exit from the combustion chamber. One shock wave will exit from the short inlet tube of the chamber before the second shock wave can exit from the tail pipe.
The momentum of the combustion products causes a partial vacuum to develop in the combustion chamber, causing a reverse flow in both the inlet and tail pipe. This reverse flow brings a new air charge into the combustion chamber where the air mixes with a new fuel charge and with the hot gas which reversed its flow in the tail pipe. The momentum of the reverse flow causes a slight compression to develop in the combustion chamber and a very vigorous mixing of the fuel, air, and hot combustion products results. Spontaneous ignition of the mixture takes place and the process repeats itself about 100 times per second.
Most pulse combustion systems today use the air valve engine because it is simple to make and has no mechanical moving parts. However, the air valve system uses a substantial percentage of the energy from the combustion process to pump the combustion air required for the succeeding fuel detonations. Even though this system can be made into an efficient thrust producing engine through the addition of thrust augmenters, the energy consumed in pumping the combustion air detracts from the total energy available for the process and reduces the overall system efficiency.
The valved type pulse combustor uses the same combustion principle except that it has a mechanical (reed, flapper, or primitive rotary type) valve on the combustion chamber inlet side which prevents any back flow of combustion products out of the inlet tube. The valve is closed during the final phase of fuel/air mixing and during the explosion, so all of the combustion products exit through the tail pipe, preceded by the shock wave. However, these valved type systems have experienced limited valve life in the hot environment of the engine inlet since the valve must open and close with each combustion cycle, which can be over 100 times per second.
A further problem with engine life which affects both types of pulse combustors is caused by the corrosive effects of the hot combustion gases. Parts of the system which experience the hottest temperatures deteriorate quickly, necessitating frequent expensive repair and replacement of those parts. Attempts to fabricate such parts from corrosion resistant material, such as high quality stainless steel or inconel, have been unsuccessful because the parts have failed due to mechanical and thermal stresses in the system. Attempts to fabricate such parts from ceramics have failed because of the prohibitive costs associated with existing technology. A major reason for the prohibitive costs of ceramic construction is that the components must be cast as thick walled sections which must then be machined to form flanges, apertures, etc. This process wastes expensive ceramic material and requires extensive labor and the use of sophisticated tooling and cutting techniques.
The effectiveness of a pulse combustion energy system depends a great deal on its operating charcteristics. For example, the operating frequency affects the rate of flow through the system (and hence drying time) of material to be dried, and the amplitude, or pressure, of the sonic shock wave must be appropriate for given material since too much pressure overdries or destroys the material while too little pressure provides inadequate drying.
Present systems operate only at the natural frequency of the pulse combustor which is set by the length of the exhaust tubes. Accordingly, the operating frequency, and hence drying rate, cannot be altered without the substantial expenditures resulting from system reconstruction. This often results in systems which will only achieve their maximum efficiency when used to dry a specific type of material. Furthermore, since the natural frequency of the pulse combustor also depends on the speed of sound, variations in temperature in the combustor will change the natural frequency. As the natural frequency deviates from the frequency of optimal performance of the pulse combustor, the amplitude of the pressure wave diminishes, increasing the drying time of some products above acceptable levels. The result is lack of uniformity and effectiveness in drying.
Another disadvantage of present systems resides in the inability to meet OSHA standards. External noise has been a major reason for industry to discount pulse combustion as a viable alternative energy source since ambient noise can exceed 120 dB.
Finally, a risk of explosion is often present because of overheating during operation, excessive fuel buildup during start-up, or combustion of the dried product when there is excessive oxygen in the drying gas stream.